1. Field of the Invention
The present invention relates to a negative electrode active material for a nonaqueous electrolyte battery, a nonaqueous electrolyte battery having such negative electrode active material, a battery pack, and a vehicle.
2. Description of the Related Art
Conventionally, titanium oxide compounds are widely used as a negative electrode material for nonaqueous electrolyte lithium secondary batteries that can be used repeatedly by charging and discharging. However, although the conventional titanium oxide compound is excellent in repeated charging and discharging characteristics, it is higher in potential to metal lithium than graphite or other carbonaceous materials, and is hence lower in capacity density per unit weight. Therefore, it is low in energy density that is one of the important properties as secondary battery. For example, conventional titanium oxide (anatase) is about 165 mAh/g in its theoretical capacity, and a lithium-titanium composite oxide system is also about 180 mAh/g in its theoretical capacity, both being much inferior to the theoretical capacity of graphite material (more than 385 mAh/g). Most of titanium oxide compounds have few equivalent sites for inserting lithium in a crystal structure, and lithium is likely to be stabilized in its structure. As a result, the effective capacity becomes low. Diffusion coefficient of lithium ion in titanium oxide is low, and thus for fast charging and discharging, the negative electrode active material is desired to have a higher lithium diffusion capability.
JP-A 2003-183030 (KOKAI) discloses a negative electrode active material for a lithium secondary battery, more specifically lithium-nickel-titanium oxide having a ramsdellite type crystal structure represented by the formula: Li2−2x/3NixTi3−x/3O7 (0<x≦0.5). This lithium-nickel-titanium oxide is combined with divalent Ni having a smaller valency than tetravalent Ti, and thus in spite of the ramsdellite type crystal structure, the correlation of Li—O is reinforced by the effect of Ni getting into Ti site. Therefore, a three-dimensional space suited to intercalation and deintercalation of lithium ion cannot be presented, and the diffusion performance of lithium ion is small. Hence, this lithium-nickel-titanium oxide cannot improve the fast charging and discharging performance, in particular, among battery properties.
On the other hand, the electrode potential of titanium oxide compound is about 1.5V on the basis of metal lithium. This electrode potential is determined by oxidation-reduction reaction between Ti3+ and Ti4+ when intercalating and deintercalating lithium electrochemically, and cannot be varied. Therefore, to achieve the fast charging and discharging performance of a secondary battery, it is important to increase the negative electrode capacity, in addition to enhancement of diffusion performance of lithium ion mentioned above.
JP-A 2004-221523 (KOKAI) relates to an electrochemical capacitor, in which the negative electrode is composed of lithium-titanium oxide having a ramsdellite type crystal structure, for example, Li2Ti3O7. This Li2Ti3O7 is excellent in cycle characteristics, low in risk of overcharging among known negative electrode materials, and is known to have excellent diffusion performance of lithium ion as compared with spinel type compound such as Li4Ti5O12. However, this compound is low in effective capacity (not more than 130 mAh/g) as compared with theoretical capacity (about 230 mAh/g), and is not sufficiently applicable to fast charging and discharging.